1. Field of the Invention
The present invention relates to a reader for so-called code marks, and more particularly to a code mark reader capable of correctly reading even those small code marks which are formed on metal surfaces by oxidation marking or engraving.
2. Description of the Related Art
Optically readable code marks such as bar codes are becoming more commonly used in various fields. With the recent increase in the amount of information to be encoded in code marks, typical bar codes with which information can be encoded in only one direction have become unable to provide a sufficient space for encoding.
Therefore, so-called binary code marks, by which information can be encoded in vertical and horizontal directions, have been put into use. As examples of this type of code mark, "carla code", "vericode" and "data code" have come into general use. Among them, the data code is disclosed in U.S. Pat. No. 4,939,354 and is designed such that individual cells of a grid pattern divided in vertical and horizontal directions are colored black or white to thereby create a code mark.
Meanwhile, the kinds of products provided with code marks have also been increasing recently. For example, code marks are put on surgical equipment, such as surgical instruments, for the purpose of inventory control of said equipment in hospitals. Since many surgical instruments are small in size, code marks are required to be correspondingly small on the order of, e.g., 2.times.2 mm in size. Furthermore, bar codes are also becoming increasingly used in books and other publications. In order not to mar the finish of bindings or covers, code marks put on books are ideally as small as possible.
In the case of the data code, a mark is taken in as an image by a camera, and the mark image in black and white is converted into a matrix comprised of 0's and 1's, thereby decoding the data written in the mark. For correct decoding of the data, therefore, black and white mark elements must be precisely and distinctly discriminated without errors and be displayed as an image. Then, even if a mark is small, a camera has to be able to take in such a small mark with correct discrimination between black and white.
When a code mark is engraved on, e.g., surgical instruments by using a laser, recesses and projections on the metal surface are taken in the form of an image by a camera and are discriminated respectively as black and white elements of the mark. In other words, unlike a mark printed in black and white, a mark in the form of recesses and projections on a metal surface, which is not originally colored, has to be recognized as a black and white image. It is thus required that, when recesses and projections on the metal surface are photographed by a camera, they are correctly discriminated and reproduced to be black and white in the form of image.
In the case of oxidation marking, because marked portions are turned black, discrimination between black and white marks in images is easier than the case of engraving marking. However, where the metal surface is finished to be, e.g., pear-skin by staining, ruggedness specific to the metal surface may be reproduced in the form of black and white images in addition to objective marks.
Another problem is that, because reflecting conditions of light are different depending on differences in the finish of the metal surface (e.g., mirror surface, pear-skin, hairline, buff and scotch), marks may not be correctly read due to halation or other reasons unless the lighting is properly set up.
Moreover, for surgical instruments, their shapes are not fixed, areas in which marks are marked are not always flat, and the distances between the levels of mark area and the camera are not constant when the instruments are put on a stage for reading one after another. Accordingly, when a fixed-focus type camera is used, the camera or the stage has to be movable for adjustment of the focus.